There are many processes available for the purpose of impregnating a preform with liquid resin in order to make a composite. These processes may be broadly characterized into two categories. One category includes wet lay-up methods while the other category features some form of resin infusion such as resin transfer molding (RTM).
Wet lay-up processes tend to have lower average quality than resin infused parts. They are labor intensive to manufacture because each layer of preform material must be individually coated with resin and carefully positioned by hand. One frequent problem with wet lay-up methods is air bubble entrapment inside the composite while the resin is being applied to the preform. A second difficulty is achieving a uniform part thickness and fiber volume fraction which ultimately influence the material properties. Another problem with wet lay-up processes is the excessive amount of fumes given off by the exposed resin before and during cure.
Resin infusion methods are carried out in a closed system which eliminates most of the fumes. Resin infusion processes can also infuse resin into a preform with a vacuum thus avoiding air bubble entrapment. Resin infusion methods allow for better control over part dimensions and fiber volume fraction. Thus, resin infusion methods overcome many of the limitations of wet lay-up processes. Unfortunately, until recently resin infusion processes tended be more expensive than wet lay-up because of the more expensive molding apparatus required. In the past, a rigid closed mold was required. Even though closed mold methods are improvements over wet lay-up methods, there are problems with closed mold methods beyond that of cost. When infiltrating high volume fraction preforms, standard closed mold methods require high resin injection pressures and long infiltration times because the low permeability of high volume fraction preforms.
More recently, several novel vacuum infusion techniques were introduced which do not require an expensive closed mold. Instead, they incorporate a single rigid mold surface upon which the preform rests. The preform is covered by a impermeable sheet and is sealed at its periphery forming a preform cavity which can be evacuated using a vacuum pump. Atmospheric pressure provides both the compaction force on the preform and also the driving force for resin infusion from an external supply into the lower pressure preform cavity. Despite the simplicity of this approach there were still problems with the infusion process because many high viscosity resins could not adequately infiltrate low permeability preforms.
The resistance to resin infiltration increases with the distance the resin must flow through the preform. In vacuum assisted resin infusion, the injection pressure cannot exceed the ambient atmospheric pressure without pressing the outer sheet away from the preform. Thus, infiltration is often slow and incomplete. Often, regions of the preform are not infiltrated with resin, while excess resin collects in undesired locations.
One clever way to overcome the problem of a low permeability preform was to artificially increase the permeability at certain locations within the preform cavity. There have been several embodiments of this fundamental concept in the art These include but are not limited to: placement of tubular arteries between adjacent preform layers, drilling an array of holes in a rigid plate which rests on the preform, and the use of a distribution medium on top of the preform. All these methods have their own limitations. Of the methods, the processes incorporating a distribution medium are the most useful. They are also the most closely related methods to those disclosed in the present patent and therefore will be described in greater detail.
In the distribution medium process, a preform is placed on a tool surface and covered with a permeable sheet. A distribution medium is placed on top of the permeable sheet and is covered with a continuous non-permeable sheet sealed at its periphery. A vacuum is drawn on the entire assembly of preform, permeable sheet, and distribution medium. Resin is introduced to the distribution medium which provides high permeability pathways for the resin to distribute itself over the entire top surface of the preform. The injection pressure, which must be less than atmospheric pressure, forces most of the resin through the permeable membrane and into the preform. Satisfactory resin impregnation is usually achieved since there is a relatively small resistance to infiltration of the preform in the thickness direction in comparison with the in-plane direction. The lower resistance to infiltration also results in shorter infusion times.
Despite the benefits of this improved resin infusion method, it still has some disadvantages which make it unattractive for many applications. When resin comes into contact with the distribution medium it remains trapped in the distribution medium and is subsequently cured. After the resin has cured, the permeable membrane, the clogged distribution medium, and the impermeable outer membrane must be separated from the composite part and discarded as waste. While the process is quite useful for proto-typing and low volume production, the amount of waste produced is incompatible with the required efficiencies of mass production.
These and other problems remain in the field of infusion molding of composite structures. In this patent we describe novel means of overcoming the aforementioned difficulties.
The molding methods and apparatus of the present invention have many different uses, and the apparatus can take on different forms. An example is provided of infiltrating an advanced composite preform, which is an assemblage of reinforcing fibers, with a liquid plastic resin. What in general could be any low permeability material will in the following disclosure be called the preform. The infiltrating fluid could be one of many different fluids, such as gases, liquids, and liquids containing suspended solid particles, which in the description are collectively referred to hereafter as resin.
The preform can be can be made from a variety of reinforcing fibers including fiberglass, Kevlar (aramid) fibers, and carbon fibers. There are many types and configurations of fibers and yams made from fibers. The yarns may be formed into many different types of assemblages including weaves, braids, and knits as well as short and long fiber mats. Different types of fibers may be combined in an assemblage to form a hybrid preform. All these and other variations are known by those skilled in the art. The preform may also include inserts of various types such as foam cores, honeycomb cores, balsa wood, metal inserts and reinforcements. Additionally, the preform may contain sensors or other devices. All of these components are known in the prior art.
The resin is any of a variety of curable liquid resins such as polyester, vinyl ester, and epoxy. The resins may be catalyzed for high temperature cure or room temperature cure and for various cures times as is compatible with the needs of the molding process. The resin should be properly catalyzed and degassed prior to injection into the preform cavity. The wide variety of useful resins and the methods of preparing the resin are all widely known in the art.
In each of the embodiments of the invention, it is desirable that a vacuum be drawn on the cavity containing the preform prior to beginning the flow of resin. In this way, the problem of entrapping an air bubble in the resin is avoided. A source of vacuum may continue to be applied to the preform cavity during the infiltration of resin and until the resin has cured or it may be removed at some point during the infiltration. Even if the source of vacuum is removed (by closing a valve for example) the space inside the preform cavity should remain substantially a vacuum during resin infiltration if the preform cavity is properly sealed. In some embodiments of the invention, it is possible to infiltrate the preform with resin without initially drawing a vacuum on the preform cavity. However, since this method introduces the possibility of trapping an air bubble in the preform and it is easy to draw a vacuum on the preform cavity, this method is not recommended.
The number and position of the sources of vacuum and resin depend on the specific application. Each resin and vacuum source is able to be controlled individually using a valve or other means of stopping the flow of material through the source. The flow of resin should be stopped when sufficient resin has entered the mold. This may be controlled by properly controlling the cure time and/or measuring the amount of resin that has entered the mold and stopping the flow of resin when sufficient resin has entered the mold.
Temporary Resin Distribution Channels
As is discussed in the Background to the Invention, the prior art contains various means of aiding the distribution of resin over the surface of a preform for the infiltration of a preform with resin. However, none of these methods provides a means of creating temporary resin distribution pathways on the surface of the preform. The advantage of temporary resin distribution channels over the use of a distribution medium is a major reduction in the amount of waste material produced. When producing composites in large quantities, this large amount of waste is costly and a source of pollution and therefore should be eliminated.
Though there are many features and embodiments of the present invention, the essence of the invention is a method and the related apparatus for molding a composite by infusing resin into a permeable preform, comprising the creation of temporary resin channels on the surface of the preform, flowing resin along the channels and from the channels into the preform, stopping the flow of resin, removing the temporary channels if desired, curing the resin in the preform and forming a composite. The methods disclosed in this patent are able to produce a composite if the temporary resin distribution channels are not removed before curing the resin. In this case, cured resin channels remain on the surface of the preform. This is not desirable in most applications and therefore, in the preferred embodiments of the invention, the resin flow channels are removed prior to curing the resin so that there are no cured resin flow channels on the surface of the composite.
There are several means of creating temporary resin distribution channels. One method comprises activating movable portions of the mold surface to create a network of resin distribution channels on the surface of the preform and deactivating them to remove the channels. These movable portions of the mold surface can be constructed in various ways. In a rigid mold, sections of the mold can be made so they are retractable. These portions can be retracted to create the resin flow channels and extended to remove the channels after sufficient resin has entered the preform cavity. The retractable sections of the rigid mold should be constructed in such a way that they do not leak resin.
Constructing retractable sections on the internal surface of the mold makes the mold more expensive than the usual mold used in RTM. However, the speed of infiltration allowed by having high permeability pathways over the surface of the preform is worth the extra cost. This method retains the advantages of closed mold RTM such as good dimensional accuracy and high fiber volume fraction while overcoming difficulties such as high injection pressures and slow infiltration of resin when infiltrating high volume fraction preforms. Also, the problem of determining the proper resin inlet and outlet positions is eliminated.
Under the present art, much effort is put into determining the time evolution of the resin flow front in order to properly place resin inlets and outlets and to determine the time required to inject resin so that the preform is fully impregnated. The use of temporary resin distribution channels essentially eliminates this problem since the time to infiltrate the preform is reduced and the progression of the flow front is easily controlled by the resin distribution network pattern. The methods used to calculate resin flow front evolution are computationally difficult and still a topic of active research.
It is desirable to avoid the problem of resin source and outlet placement entirely by using the resin distribution channels to control the flow front progression. Optimization of the inlet and outlet positions requires repeated calculations and/or experimental trial and error. Both of these tasks are expensive. The network of resin distribution channels provides rapid resin infusion without complicated calculations or expensive trial and error experimentation. In the case of high volume production, the increased speed of resin infusion, the shorter mold development times, and the lower risk of making an unsatisfactory mold are all worth the extra cost of the moveable mold channels.
Another method of creating temporary resin distribution channels is to have mechanically movable portions of the mold surface which are made of a stiff elastomeric material. A means of deforming the stiff elastomeric material into resin distribution channels is provided in the body of the mold. This embodiment of the invention provides a simple, cost effective means of creating temporary resin distribution channels in the surface of the mold. After resin has infiltrated the preform, the stiff elastomeric portions of the mold are pushed into a position flush with the surface of the preform. At this point, curing of the resin may begin.
Instead of deforming or moving portions of rigid molds, a thin flexible membrane can be placed over the preform and portions of the membrane can be caused to deform into resin distribution channels by various means. The various embodiments of this general class of molding apparatus have advantages over the previous embodiments in terms of the cost and ease of making and preparing the mold.
One of the means of deforming the flexible impermeable membrane involves applying mechanically or electro-mechanically induced forces at distributed points along the surface of the flexible membrane. These forces cause the membrane to move away from the surface of the preform temporarily to allow the resin to flow over the surface of the preform.
There are various embodiments of the invention which use an impermeable flexible membrane which is temporarily deformed into resin distribution channels on the surface of a preform. One set of these embodiments relies on a differential pressure between the preform side of the flexible membrane and the other side of the membrane. By properly applying a higher pressure on the preform side of the membrane than the other side of the membrane, the flexible membrane will deform into temporary resin distribution channels. When the proper amount of resin has infiltrated the preform, a source of fluid (usually air) at the proper pressure is applied to the channels in the mold surface. The applied pressure should be greater than the pressure of the resin under the flexible membrane. In this way the resin distribution channels on the surface of the preform are collapsed and the flexible membrane presses down evenly and smoothly over the preform. In fact, the pressure of the resin plus the pressure compressing the preform is equal to the pressure on the other side of the flexible membrane. Thus, by adjusting the amount of resin in the preform and the applied external pressure, the amount of preform compression can be controlled within certain limits.
One method of applying a differential pressure between the two sides of the flexible membrane is to cover the flexible membrane with a mold which contains a network of channels in its surface. Resin is then introduced to the preform side of the flexible membrane at a pressure higher than the pressure on the other side of the membrane. This causes the membrane to be forced out and away from the preform and the membrane then takes on the shape of the internal surface of the containing mold. Since this containing mold has a network of channels in its surface, the flexible membrane now has created a network of resin distribution channels on the surface of the preform.
In one embodiment of the invention which uses a flexible impermeable membrane covered by a mold with internal surface channels, the mold is made from a rigid sheet material with channels impressed upon its surface. The rigid sheet material may be sheet metal or other suitable rigid sheet materials which can be formed into the proper shape. As with the majority of the embodiments of the invention described in this disclosure, it is desirable to maintain a vacuum in the cavity containing the preform. Additionally, it is desirable in this embodiment to maintain a vacuum in the channels in the surface of the rigid sheet above the flexible membrane. Thus, before resin is introduced, there is a vacuum on both sides of the flexible membrane and therefore, the flexible membrane does not deform. Additionally, the ambient atmospheric pressure presses down upon the rigid sheet which in turn presses down on the flexible membrane in regions where there is not a channel. These regions of the flexible membrane in turn press down upon the preform compacting it to a high fiber volume fraction. When liquid resin is introduced under the flexible membrane, the resin presses the flexible membrane into the channels in the surface of the rigid sheet thereby creating temporary resin distribution channels on the surface of the preform. These high permeability channels allow the resin to easily and quickly spread out over the preform and infiltrate the preform quickly and completely. Since all points in the preform are only relatively short distances from a channel in a properly design channel network, the resin has no difficulty reaching all points in the preform. After resin has impregnated the preform, the vacuum in the channels is removed and atmospheric pressure is applied to the channels forcing the flexible membrane against the preform.
In another embodiment of the flexible impermeable membrane covered by a mold with internal surface channels, the mold is made from thick rigid material with a network of channels formed in its surface. The rigid material may be a strong material such as steel. This embodiment provides greater dimensional accuracy for the composite. Also, the compacting pressure is not limited to atmospheric pressure. Higher resin injection pressures are possible since the thick and rigid material can support the flexible membrane. Thick and rigid molds are more expensive and difficult to make than rigid sheets with channels. This is especially true for very large parts. Thus, both embodiments are useful and the specific application will determine which embodiment is most appropriate.
Each of the previous two embodiments of the invention require that the pressure on the preform side of the flexible membrane exceed the pressure on the other side of the membrane. The preferred method of creating this situation is to apply a source of vacuum to the channels in the internal surface of the mold. It is possible to include a layer of porous material known in the art as breather material between the mold surface and the flexible membrane to aid in quickly and evenly evacuating the channels.
An alternate method of causing the flexible membrane to deform into temporary resin distribution channels which still relies on a differential pressure between the preform side of the flexible membrane and the other side of the membrane but does not use a mold with a network of channels in its surface is to use a spacer system. A spacer system is a system of sturdy shapes inter-connected in such a way that they provide a network of gaps into which the flexible membrane can deform to create temporary resin distribution channels when the pressure on the preform side of the flexible membrane exceeds the pressure on the other side of the membrane where the spacer systems is located.
There are many appropriate forms of the spacer system. One form, called a link mat, is a geometric array of spacer blocks connected to each other with link rods. The blocks and link rods can be made of plastic or other suitably sturdy material. The link mat is placed on top of the flexible impermeable membrane which is in contact with the preform and then covered with an additional flexible impermeable membrane which is able to be sealed against the first membrane. The purpose of the second membrane is to allow a reduced pressure to be applied to the cavity containing the spacer system.
Another form of spacer system is similar to a woven mat which is also covered with an impermeable outer cover. The mat consists of large flexible rods in the weft direction held together with thin yams in the warp direction. This arrangement produces a series of channels between the larger rods. The flexible membrane deforms into these channels during resin infusion and the resin distribution channels formed under the membrane and on the surface of the preform aid in flowing resin over the surface of the preform. When higher pressure fluid such as air is introduced to the cavity containing the fabric mat, the impermeable membrane between the preform and the fabric mat is pushed smoothly and evenly against the preform.
Yet another form of the spacer system is a bubble mat or inflatable bubble mat. A bubble mat is a flexible sheet with an array of bubbles on its surface. An inflatable bubble mat has a series of small inter-connections which allow the bubbles to be inflated or deflated. In both cases, the gaps between the bubbles provide a network of channels for the flexible membrane in contact with the preform to deform into when the pressure on the preform side of the flexible membrane is higher than the pressure on the bubble mat side.
The spacer systems discussed above may also contain a layer of breather material to aid in quickly and evenly adjusting the pressure in the spacer system cavity.
In one embodiment, the flexible membrane and the spacer system is combined into one unit. An inflatable bubble mat is placed on a preform which is resting on a tool surface. The bubble mat is sealed at its periphery on the tool surface so that a vacuum can be drawn on the preform cavity. The bubbles in the bubble mat are then inflated which causes the bubble mat to lift away from the preform and thereby create a system of resin distribution channels on the surface of the preform. Resin is injected under the bubble mat and flows through the channels and from the channels into the preform. When sufficient resin has entered the preform cavity, the source of resin is stopped and the bubbles are deflated allowing the bubble mat to press down smoothly and evenly over the surface of the preform.
In the embodiments of the invention disclosed so far, the temporary resin distribution channels have been created by either mechanically moving sections of the mold surface or deforming a flexible impermeable membrane by some means. In another set of embodiments of the invention, the temporary channels on the surface of the preform comprise a flexible mold material containing interior channels below the surface. A resin pressure on the preform side of the flexible mold greater than the pressure in the channels below the surface of the flexible mold causes the flexible mold material between the channels and the preform to deform into the interior channels and thereby create resin distribution channels on the surface of the preform. It is preferable in this embodiment of the invention that a vacuum be drawn on the cavity containing the preform and that a vacuum also be drawn on the channels below the surface of the flexible mold material during resin infusion. After a sufficient amount of resin has entered the preform, the source of resin and vacuum are stopped and a pressure higher than the pressure of the resin in the preform cavity is applied to the channels. This causes the material between the preform and the channels to move toward the preform and become flush with the surface of the preform. Curing the resin after the channels have been removed provides a composite with a smooth surface.